The present invention relates to a molding die for an optical element, the optical element and its master die, and particularly to a molding die for the optical element by which a desired optical surface can be easily formed and the dimension and accuracy are increased, the optical element molded thereby, and a master die for molding it.
As a production method of the molding die for the optical element of a plastic optical element which is generally conducted, for example, a blank (primary processed goods) is made by a steel material or stainless steel, and by a chemical plating called electroless plating, an alloy composed of amorphous-like nickel and phosphorous is coated in the thickness of about 100 μm thereon, and this plating layer is cutting-processed by a diamond tool by a super-precision processing machine, and a highly accurate optical surface is obtained.
According to the method of the conventional technology, basically, because the part shape is formed by mechanical processing, the part accuracy is easily increased to near the motion accuracy of the processing machine. However, the conventional technology has the following disadvantages: mechanical processing and chemical processing are mixed in the production process, it is troublesome, it takes a long time to produce, it is, necessary to make a blank (primary processed goods), the plating processing is not necessarily stable, and the adherence strength of the plating layer fluctuates according to a deviation or stained condition of the composition of the blank, which generates a pinhole-like defect called a pit. Moreover, because, the optical molding surface corresponding to the optical surface of the optical element must be formed within the thickness of the plating layer, there is no margin in the plating thickness when the optical molding surface is processed again, and sometimes the processing is impossible, and generally, the repeated use can not be conducted, and the molding die cost is high.
Further, when the optical molding surface requires much processing, the shape of the optical molding surface finished by cutting processing fluctuates slightly due to a change of a condition of cutting edge of a tool, processing condition, or processing environmental temperature. In this processing fluctuation of the optical molding surface, about 100 nm optical surface shape error is generally generated, and even when it is processed with very great care, about 50 nm error remains, which becomes is a processing accuracy limit.
Further, recently, an optical system in which the chromatic aberration is efficiently corrected by providing a diffraction groove on an optical surface, comes to practical use in an optical information recording field, and is produced in a large amount. As an optical material, plastic or glass is used, and in an ultra red optical system, a crystal material such as ZnSe is also used. An effective method when such an optical element is produced in a large amount, is molding, and at this time, a technology by which an optical molding surface having a fine diffraction groove of the molding die is efficiently produced with the high accuracy, is very important.
For example, when a fine pattern having an optical function such as a diffraction groove on the optical molding surface is formed by diamond cutting, the sharpness of the tool edge influences on the accuracy of the shape of the diffraction groove, and when it is transferred as the optical surface, it is well known as described in Japanese Tokkai No. 2001-195769 that it greatly influences the diffraction efficiency.
Accordingly, in order not to lower the diffraction efficiency of a diffraction ring band, it is necessary to sufficiently reduce the dimension of an edge of the tool is enough reduced. Accordingly, because the cutting resistance is concentrically burdened onto the small tool edge portion, it is necessary to reduce the cutting depth amount, and the greatly increasing the number of processing cycles until the whole of the optical molding surface is uniformly cut and removed. Further, in order to prevent the deterioration of the surface roughness of the optical molding surface by a small cutter mark, it is necessary to slow the feed speed, and increase the optical molding surface processing time per cycle. As a result, because the cutting length is increased, the wear of the tool edge is increased, and the tool exchange is frequent. That is, when the optical molding surface having a fine shape is processed by the conventional diamond cutting, the life of the tool becomes very short, and carbons in the diamond by the cutting is dispersed in the blank, thereby, the life of the tool is more reduced. Further, the time to process one optical molding surface is also increased, the processing efficiency is very lowered, and the productivity of the molding die is lowered, resulting in a sudden increase of the cost. Therefore, particularly when the optical molding surface having the fine shape on the surface is finished by the diamond cutting, a simple and short delivery date molding die production method is desired.
In addition, recently, it is tried that a new optical function is added to an optical element in such a manner that a fine structure having the dimension from several times of the wavelength to the dimension smaller than that, is provided on the optical surface. For example, the diffraction groove is provided on the surface of the molded lens having the aspheric optical surface, and the ordinary light converging function by the refraction and the positive dispersion which is generated as the side reaction at the time are cancelled by utilizing the negative dispersion in which the diffraction by the diffraction groove is large, and originally, a single lens optical element having the achromatic function which is impossible by only the refraction, is brought into practical use in a pick-up objective lens for a DVD/CD exchangeable optical disk. This is a lens in which the diffraction action by the diffraction groove having the dimension of several 10 times of the wavelength of the light transmitting the optical element is utilized, and such an area to process the diffraction action by the sufficiently larger structure than the wavelength is called a scalar area.
On the one hand, when a protrusion of the conical shape is formed in close formation on the surface of the optical surface at fine intervals which is one several numbers of the wavelength of the light transmitting the optical element, it is well known that the reflection suppression function of the light can be made to exhibit. That is, when a protrusion is formed at a fine interval, the refractive index change at the air interface when the light wave is incident on the optical element is changed not as in the conventional optical element in which it is instantaneously changed from 1 to the medium refractive index, but changed moderately, thereby, the reflection of the light can be suppressed. The surface on which such a protrusion is formed, is a fine structure called a moth eye, and when the finer structural bodies than the wavelength of the light are aligned at shorter periods than the wavelength, the individual structure does not even diffract, and it acts as an average refractive index on the light wave, and an area on which such an action is worked, is generally called an equivalent refractive index area. Relating to such an equivalent refractive index area, it is described in, for example, the Institute of Electronics, Information and Communication Engineers paper magazine, C Vol. J83-C No. 3 pp. 173-181 March 2000.
According to such fine structure of an equivalent refractive index area, it is considered that it has also the merits in the production as the following: while the angle dependency or wavelength dependency of the reflection prevention effect is more reduced than the conventional reflection prevention coat, simultaneously the large reflection prevention effect can be obtained, further, because the optical surface and fine structure can be simultaneously formed by the molding, the lens function and reflection prevention function can be simultaneously obtained, and eliminating the need for after-processing such as the conventional coat processing after molding. Further, when the fine structure of such an equivalent refractive index area is arranged in such a manner that it has the directionality to the optical surface, the strong optical anisotropy can be made so that the optical surface has it, and the double refraction optical element which is conventionally produced by cutting the crystal such as a quartz can be obtained by the molding. When it is combined with the refraction optical element or reflection optical element, a new optical function can be added. The optical anisotropy in this case is called the structure double refraction.
Between the above-described scalar area and equivalent refractive index area, there is a resonance area in which the diffraction efficiency is suddenly changed due to a slight difference of the incident condition. For example, when a diffraction groove width is reduced, a phenomenon (anomaly) in which the diffraction efficiency is suddenly reduced at about several times of the wavelength, further, is increased, is generated. The same effect as the ordinary interference filter, as the guide wave mode resonance lattice filter by which only a specific wavelength is reflected, when such a diffraction groove width is adjusted, can be realized by more reducing the angle dependency.
In this connection, when an optical element is formed by utilizing the scalar area, equivalent refractive index area, or resonance area, it is necessary that fine protrusions (or hollows) are formed on the optical surface. When the optical element having such fine protrusions (hollows) is mass-produced, generally, it is appropriate that the molding is conducted by using the plastic as the raw material, but in such a case, it is necessary that the optical molding surface provided with the hollow (or protrusion) corresponding to the fine protrusion (or hollow) is provided on the molding die.
However, relating to the above-described protrusion (or hollow) of the equivalent refraction area or resonance area, it is necessary that the protrusion (or hollow) is formed at an interval of several tens to several hundreds nm, and it is very difficult to obtain by mechanical processing including the cutting processing, and in fact a practical molding die has not yet been produced. In addition, there is also a problem in which the re-use of the conventional molding die is difficult.